1. Field of the Invention
The present invention generally relates to devices for testing signals in cables, such as telecommunications signals, and more particularly to a device for testing the operation of an optical fiber which may be carrying an active signal.
2. Description of the Prior Art
In fiber optic networks, such as are used for telecommunications or cable television, it often becomes necessary to access the fibers and examine any signal therein, for example, to ensure that there are not significant faults (optical losses) in the network, or to identify and redirect "miswired" fibers. Several instruments have been devised which extract any light signal from a fiber, or inject light therein (collectively, "tapping" the fiber), without cutting into or otherwise damaging the fiber (i.e., non-intrusive measurement), by introducing a microbend in the fiber and positioning a photoelectric eye near the bend, to sense any leakage of light through the cladding and buffer of the fiber. Industry standards demand an optical loss at the bend of no more than 3 dB. These instruments are commonly referred to as "pick-off" or "clip-on" devices, and are used with fiber identifiers, power meters, active devices, etc., as shown in the following patents:
U.S. Pat. No. 4,586,783 PA1 U.S. Pat. No. 4,728,169 PA1 U.S. Pat. No. 4,747,652 PA1 U.S. Pat. No. 4,759,605 PA1 U.S. Pat. No. 4,790,617 PA1 U.S. Pat. No. 4,824,199 PA1 U.S. Pat. No. 4,834,482 PA1 U.S. Pat. No. 4,981,334 PA1 U.S. Pat. No. 5,138,690 PA1 U.S. Pat. No. 5,315,365 PA1 U.S. Pat. No. 5,315,675 PA1 Patent Cooperation Treaty Application No. GB88/00225 PA1 Patent Cooperation Treaty Application No. GB92/01152 PA1 European Patent Application No. 325,382 PA1 Japanese Patent Application (Kokai) No. 60-79244
Several of the devices shown in these patents disclose means for aligning the optical fiber with a mandrel or sensor head having a groove or other surface structures which position the fiber adjacent the photodiode and induce the bend. While such means improve the repeatability of measurements taken with the clip-on device, industry standards have become more exacting and the existing clip-on mechanisms do not have the repeatability currently demanded by telecommunications and cable operators. This limitation is due in part to the prohibitive cost of mass manufacturing clip-on components having extremely tight tolerances (on the order of 6 microns), and also due to the lack of a precise method of positioning or holding the clip-on device in a stable manner, prior to placement of the fiber in the sensor head, as well as during the measurement process. Indeed, while the foregoing references disclose means for aligning the mandrel with the sensor head, there is no disclosure of any support structure for the optical fiber, separate from the clip-on device, or any means for precisely aligning the clip-on housing with any such support structure. In other words, when the clip-on device is completely disengaged from the fiber and then reapplied, there is no assurance that the same portion of the fiber will be tested, or that the fiber will have the same torsional and compressive loads, etc.
One clip-on device 10 that improves on these designs, and is probably the closest prior art to the present invention, is adapted to fasten onto a fiber support tray 12 as shown in FIG. 1. Tray 12 has an aperture 14 therein over which passes a fiber 16. A wall 18 surrounds aperture 14, and notches 20 are formed in wall 18 to allow the straight passage of the fiber across aperture 14. Clip-on device 10 has a head portion 22 in which resides an optical sensor, i.e., photodiode. Head 22 includes a fitting or shroud 24 which serves to block out ambient light from above the tray. As the handles of the device are squeezed and shroud 24 forcibly contacts tray 12, a mandrel in the lower housing of the device pushes fiber 16 into a fiber guide 26 in head 22 and the measurement is taken by engaging a switch 27 (a slight delay may be provided in the measurement to ensure that the reading is more accurate and repeatable). Fiber guide 26 also provides nominal alignment with wall 18, i.e., the inner surface of wall 18 has a rectangular cross-section and is sized to fit the rectangular outer shape of fiber guide 26. Fiber guide 26 may be provided with a rectangular cutout to further align the mandrel with the waveguide.
The prior an clip-on device 10 is shown in detail in the exploded perspective of FIG. 2, and generally includes a handle 28 supporting both head 22 and a mandrel assembly 30, and a lever 32 for actuating the device by forcibly urging mandrel assembly 30 toward head 22. Head 22 has a cavity therein which receives a waveguide element 34 constructed of an optically transmissive material (such as acrylic) and having one or more optical sensors (not shown) integrated therein for detecting leakage of any light from a fiber placed against the element. The outer walls of the element may be painted (e.g., black), to further reduce the effect of ambient light and also reduce internal reflections within the waveguide. Output wires (not shown) from the sensors lead to a connector or adapter 36 formed on handle 28, which receives a retractable cord 38. Adapter 36 preferably includes some strain relief structures. Cord 38 leads to the power meter (not shown) which interprets the detector information and converts it to, e.g., a decibel reading displayed on a liquid crystal display. Head 22 is attached to the upper arm 40 of handle 28.
Mandrel assembly 30 includes a cylindrical member 42 which houses a shaft 44 on which is mounted a mandrel 46. Cylinder 42 is fixedly attached to the lower arm 48 of handle 28 by means of a retaining ring 50 which abuts an annular flange 52 on cylinder 42. Shaft 44 is allowed to slide within cylinder 42, by means of a bearing 54. A tubular housing or button 56 slides on the outside of cylinder 42, i.e., button 56 has an inner diameter which is approximately equal to the outer diameter of cylinder 42. Lever 32, which is pivotally attached at one end to handle 28, is coupled to button 56 by actuator pins 58 which engage grooves 60 formed on opposing sides of button 56. In this manner, when lever 32 is squeezed toward handle 28, actuator pins 58 push button 56 upward, but the movement of the button (and shaft 44) is linear rather than arcuate due to the ability of pins 58 to slide in grooves 60. Button 56 (which may also be directly actuated by pushing on the button rather than by squeezing lever 32) forcibly urges shaft 44 upward. Return spring 62, which fits in an interior annular groove of cylinder 42, maintains cylinder 42 in a retracted position when device 12 is in its unactuated state. Spring 66 urges the mandrel housing upward. The force applied by lever 32 to mandrel 46 is further transmitted through another spring 68, abutting a second washer or flange 70 on shaft 44, i.e., mandrel 46 may slide within the upper portion of shaft 44. Compression of spring 68 determines the total force which can be applied by mandrel 46 to the fiber, and more particularly prescribes a set pressure level on the fiber, independent of operator induced loading beyond the initial load requirements. Flanges 64 and 70 provide solid contact between button 56, shaft 44 and the mandrel housing. Stop pins 72 also provide means for absolutely limiting the movement of mandrel 46 in the retracted position, while cross-pin 73 restrains rotational movement. Shaft 44 can move back about 2 mm from this extreme position before the force of the fiber is affected. Mandrel 46 is further aligned with fiber guide 26 by mandrel housing 74 which fits in the upper portion of cylinder 42. Guide wires 76 are used, in conjunction with longitudinal grooves 78 formed in the outer surface of mandrel housing 74, to properly seat mandrel housing 74 in cylinder 44. The use of the springs allows mandrel housing 74 to move into position first with mandrel 46 progressing toward the fiber after mandrel housing 74 has come to rest.
A latching mechanism is provided to secure device 10 in a stowed position when it is not being used. This mechanism includes a latching cylinder 80 having inner flanges 82 which are adapted to contact lugs 84 on the outer surface of mandrel housing 74. Prior to storage of the device, lever 32 is squeezed, without there being any fiber tray or other obstruction between head 22 and latching cylinder 80, raising mandrel housing 74 to engagement with fiber guide 26. This places mandrel housing 74 substantially above latching cylinder 80, such that twisting of cylinder 80 positions flanges 82 under lugs 84. Lever 32 may then be released, but mandrel housing 74 will remain locked against head 22, protecting waveguide element 34 as well as mandrel 46 from accidental damage or foreign contaminants. The latch will not permit closure of the device while the mandrel is in contact with the fiber, or allow closing of the device when it is attached to tray 12.
The use of fiber guide 26 to position head 22 prior to engagement of the fiber provides some improvement in the repeatability of measurements taken with clip-on device 10, but there is still a noticeable amount of movement between head 22 and tray 12 when the device is held on the tray, due to the manufacturing tolerances designed into the shapes of wall 18 and fiber guide 26. One prior art reference has attempted to provide alignment of the clip-on device with a fiber support structure, but that device suffers from the same limitations regarding manufacturing tolerances. Specifically, European Patent Application No. 326,250 discloses the use of four (straight) latching posts formed on the fiber support structure which mate with holes formed in the bottom of the clip-on device. Although these posts again provide nominal alignment of the device with the fiber support structure, it is difficult/costly to fabricate the posts and the clip-on device with the tolerances necessary to achieve essentially total restriction of movement of the clip-on device in any direction other than the axes of the posts.
There is also a relatively significant amount of play between mandrel 46 and fiber guide 26 of device 10. While these components could be fabricated with extremely tight tolerances and provide even further improvement in repeatability, the manufacturing techniques required for mass-producing clip-on devices would make them commercially unfeasible due to cost. It would, therefore, be desirable to devise a clip-on device which has greater accuracy and repeatability imparted by improved alignment of the mandrel with the fiber guide, as well as by improved alignment of the head of the clip-on device with the fiber support structure. It would further be advantageous if the clip-on device did not have to rely on manually securing the device to the fiber support structure during the measurement process since such manual assistance can affect the repeatability of the measurements.